Arrays of photosensitive diodes are used in an assortment of applications including, but not limited to, radiation detection, optical position encoding, and low light-level imaging, such as night photography, nuclear medical imaging, photon medical imaging, multi-slice computer tomography (CT) imaging, and ballistic photon detection etc. Typically, photodiode arrays may be formed as one- or two-dimensional arrays of aligned photodiodes, or, for optical shaft encoders, a circular or semicircular arrangement of diodes.
One problem with detection devices is that they are susceptible to various radiation damage mechanisms, such as displacement damage resulting in total dose effects and ionization damage resulting in bulk effects. Both mechanisms adversely affect the performance of detectors, transistors and integrated circuits.
Certain detector characteristics that are most affected include detector leakage current, doping characteristics, charge collection, and carrier lifetime. Over time, detectors show an increased reverse-bias current and increased forward voltage drop due to radiation damage. Further, a change in doping level, due to radiation damage, adversely affects the width of the depletion region, i.e. the voltage required for full depletion and a decrease in carrier lifetime results in signal loss as carriers recombine while traversing the depletion region.
Another disadvantage with conventional detection devices is the amount and extent of crosstalk that occurs between adjacent detector structures, primarily as a result of minority carrier leakage current between diodes. The problem of crosstalk between diodes becomes even more acute as the size of detector arrays, the size of individual detectors, the spatial resolution, and the spacing of diodes are reduced.
In certain applications, it is desirable to produce optical detectors having small lateral dimensions and spaced closely together. For example in certain medical applications, it would beneficial to increase the optical resolution of a detector array in order to permit for improved image scans, such as computer tomography scans. However, at conventional doping levels utilized for diode arrays of this type, the diffusion length of minority carriers generated by photon interaction in the semiconductor is in the range of at least many tens of microns, and such minority carriers have the potential to affect signals at diodes away from the region at which the minority carriers were generated. Therefore, the spatial resolution obtainable may be limited by diffusion of the carriers within the semiconductor itself, even if other components of the optical system are optimized and scattered light is reduced. Additionally, conventional front surface contact photodiodes require greater space devoted to electrical contacts and, therefore, limit the density of photodiodes in a given application.
Various approaches have been used to minimize such crosstalk including, but not limited to, providing inactive photodiodes to balance the leakage current, as described in U.S. Pat. Nos. 4,904,861 and 4,998,013 to Epstein et al., the utilization of suction diodes for the removal of the slow diffusion currents to reduce the settling time of detectors to acceptable levels, as described in U.S. Pat. No. 5,408,122, and providing a gradient in doping density in the epitaxial layer, as described in U.S. Pat. No. 5,430,321 to Effelsberg.
Additionally, certain applications require the placement of electrical contacts in a manner that does not obstruct an illuminating surface, such as with front illuminated photodiode arrays. U.S. Pat. Nos. 6,510,195 and 6,426,991 attempt to disclose a top-surface photodiode array. However, these disclosures fail to provide sufficient teachings to inform one of ordinary skill in the art how to manufacture such modified top-surface photodiode arrays.
Despite attempts to improve the overall performance characteristics of photodiode arrays and their individual diode units, within detection systems, photodiode arrays capable of reducing crosstalk while being less susceptible to radiation damage are still needed. Additionally, there is need for a semiconductor circuit and an economically feasible design and fabrication method so that it is capable of improving the spatial resolution of detectors integrated therein.